The rat mammary epithelial stem cell line, Rat Mammary 25, and the dimethyl sulfoxide-resistant variant derived from it, Rama 259, when injected separately into female, virgin, athymic nude mice formed tumors which grew at approximately the same rate. When the mice were mated, the growth rate of the Rama 259 tumors increased 2.5-fold during the pregnancy and lactation, while that of the Rama 25 tumors was unaffected. The tumors in all cases consisted of glandular-like structures and blocks of cuboidal and elongated cells. Antibodies were raised against the three major caseins in rat milk. Examination of histological sections showed that antibodies immunocytochemically stained some of the cells (5 to 20%) in the glandular regions, a few (1 to 5%) of the cuboidal cells, and none of the elongated cells in Rama 25 tumors from lactating mice. Only a relative few (1 to 5%) of the glandular cells stained in Rama 25 tumors from perphenazine-treated mice, while no cells (<1%) stained in tumors from virgin mice. The casein antibodies failed to stain any cells (<1%) in the Rama 259 tumors even when they stained luminal epithelial cells in the mammary glands taken from the same lactating or perphenazine-treated mice. These results demonstrated that a minor population of cells within the Rama 25 tumors can be hormonally regulated to produce casein in a way similar to that for the majority of the cells in normal mammary glands.